The International Classification of Diseases (ICD) is the international "standard diagnostic tool for epidemiology, health management and clinical purposes." Its full official name is International Statistical Classification of Diseases and Related Health Problems.[1]

The ICD is maintained by the World Health Organization (WHO), which is the directing and coordinating authority for health within the United Nations System.[2] The ICD is originally designed as a health care classification system, providing a system of diagnostic codes for classifying diseases, including nuanced classifications of a wide variety of signs, symptoms, abnormal findings, complaints, social circumstances, and external causes of injury or disease. This system is designed to map health conditions to corresponding generic categories together with specific variations, assigning for these a designated code, up to six characters long. Thus, major categories are designed to include a set of similar diseases. ICD-11 is a major step forward, because it has the necessary terminological and ontological elements for seamless use in digital health.

The ICD is published by the WHO and used worldwide for morbidity and mortality statistics, reimbursement systems, and automated decision support in health care. This system is designed to promote international comparability in the collection, processing, classification, and presentation of these statistics. Like the analogous Diagnostic and Statistical Manual of Mental Disorders (which is limited to psychiatric disorders and almost exclusive to the United States), the ICD is a major project to statistically classify all health disorders, and provide diagnostic assistance. The ICD is a core statistically based classificatory diagnostic system for health care related issues of the WHO Family of International Classifications (WHO-FIC).[3]

The ICD is revised periodically and is currently in its 10th revision. ICD-10, as it is therefore known, is from 1992 and the WHO publishes annual minor updates and triennial major updates. The final draft of the ICD-11 system is expected to be submitted to WHO's World Health Assembly (WHA) for official endorsement in 2019.[4] The version for preparation of approval at the WHA was released on 18 June 2018.[4][5]

A number of countries adopted Bertillon's system, which was based on the principle of distinguishing between general diseases and those localized to a particular organ or anatomical site, as used by the City of Paris for classifying deaths. Subsequent revisions represented a synthesis of English, German, and Swiss classifications, expanding from the original 44 titles to 161 titles. In 1898, the American Public Health Association (APHA) recommended that the registrars of Canada, Mexico, and the United States also adopt it. The APHA also recommended revising the system every 10 years to ensure the system remained current with medical practice advances. As a result, the first international conference to revise the International Classification of Causes of Death took place in 1900, with revisions occurring every ten years thereafter. At that time, the classification system was contained in one book, which included an Alphabetic Index as well as a Tabular List. The book was small compared with current coding texts.

The revisions that followed contained minor changes, until the sixth revision of the classification system. With the sixth revision, the classification system expanded to two volumes. The sixth revision included morbidity and mortality conditions, and its title was modified to reflect the changes: International Statistical Classification of Diseases, Injuries and Causes of Death (ICD). Prior to the sixth revision, responsibility for ICD revisions fell to the Mixed Commission, a group composed of representatives from the International Statistical Institute and the Health Organization of the League of Nations. In 1948, the WHO assumed responsibility for preparing and publishing the revisions to the ICD every ten years. WHO sponsored the seventh and eighth revisions in 1957 and 1968, respectively. It later became clear that the established ten year interval between revisions was too short.

The ICD is currently the most widely used statistical classification system for diseases in the world.[8] In addition, some countries—including Australia, Canada, and the United States—have developed their own adaptations of ICD, with more procedure codes for classification of operative or diagnostic procedures.

The ICD-6, published in 1949, was the first to be shaped to become suitable for morbidity reporting. Accordingly, the name changed from International List of Causes of Death to International Statistical Classification of Diseases. The combined code section for injuries and their associated accidents was split into two, a chapter for injuries, and a chapter for their external causes. With use for morbidity there was a need for coding mental conditions, and for the first time a section on mental disorders was added .[9][10]

The international Conference for the Seventh Revision of the International Classification of Diseases was held in Paris under the auspices of WHO in February 1955. In accordance with a recommendation of the WHO Expert Committee on Health Statistics, this revision was limited to essential changes and amendments of errors and inconsistencies.[10]

The 8th Revision Conference convened by WHO met in Geneva, from 6 to 12 July 1965. This revision was more radical than the Seventh but left unchanged the basic structure of the Classification and the general philosophy of classifying diseases, whenever possible, according to their etiology rather than a particular manifestation.
During the years that the Seventh and Eighth Revisions of the ICD were in force, the use of the ICD for indexing hospital medical records increased rapidly and some countries prepared national adaptations which provided the additional detail needed for this application of the ICD.
In the USA, a group of consultants was asked to study the 8th revision of ICD (ICD-8a) for its applicability to various users in the United States. This group recommended that further detail be provided for coding hospital and morbidity data. The American Hospital Association's "Advisory Committee to the Central Office on ICDA" developed the needed adaptation proposals, resulting in the publication of the International Classification of Diseases, Adapted (ICDA). In 1968, the United States Public Health Service published the International Classification of Diseases, Adapted, 8th Revision for use in the United States (ICDA-8a). Beginning in 1968, ICDA-8a served as the basis for coding diagnostic data for both official morbidity [and mortality] statistics in the United States.[10][11]

The International Conference for the Ninth Revision of the International Statistical Classification of Diseases, Injuries, and Causes of Death, convened by WHO, met in Geneva from 30 September to 6 October 1975. In the discussions leading up to the conference, it had originally been intended that there should be little change other than updating of the classification. This was mainly because of the expense of adapting data processing systems each time the classification was revised.

There had been an enormous growth of interest in the ICD and ways had to be found of responding to this, partly by modifying the classification itself and partly by introducing special coding provisions. A number of representations were made by specialist bodies which had become interested in using the ICD for their own statistics. Some subject areas in the classification were regarded as inappropriately arranged and there was considerable pressure for more detail and for adaptation of the classification to make it more relevant for the evaluation of medical care, by classifying conditions to the chapters concerned with the part of the body affected rather than to those dealing with the underlying generalized disease.[7]

At the other end of the scale, there were representations from countries and areas where a detailed and sophisticated classification was irrelevant, but which nevertheless needed a classification based on the ICD in order to assess their progress in health care and in the control of disease. A field test with a bi-axial classification approach—one axis (criterion) for anatomy, with another for etiology—showed the impracticability of such approach for routine use.[citation needed]

The final proposals presented to and accepted by the Conference in 1978[12] retained the basic structure of the ICD, although with much additional detail at the level of the four digit subcategories, and some optional five digit subdivisions. For the benefit of users not requiring such detail, care was taken to ensure that the categories at the three digit level were appropriate.

For the benefit of users wishing to produce statistics and indexes oriented towards medical care, the 9th Revision included an optional alternative method of classifying diagnostic statements, including information about both an underlying general disease and a manifestation in a particular organ or site. This system became known as the dagger and asterisk system and is retained in the Tenth Revision. A number of other technical innovations were included in the Ninth Revision, aimed at increasing its flexibility for use in a variety of situations.[citation needed]

It was eventually replaced by ICD-10, the version currently in use by the WHO and most countries. Given the widespread expansion in the tenth revision, it is not possible to convert ICD-9 data sets directly into ICD-10 data sets, although some tools are available to help guide users.[13]
Publication of ICD-9 without IP restrictions in a world with evolving electronic data systems led to a range of products based on ICD-9, such as MeDRA or the Read directory.[10][11]

ICPM

When ICD-9 was published by the World Health Organization (WHO), the International Classification of Procedures in Medicine (ICPM) was also developed (1975) and published (1978). The ICPM surgical procedures fascicle was originally created by the United States, based on its adaptations of ICD (called ICDA), which had contained a procedure classification since 1962. ICPM is published separately from the ICD disease classification as a series of supplementary documents called fascicles (bundles or groups of items). Each fascicle contains a classification of modes of laboratory, radiology, surgery, therapy, and other diagnostic procedures. Many countries have adapted and translated the ICPM in parts or as a whole and are using it with amendments since then.[10][11]

ICD-9-CM

International Classification of Diseases, Clinical Modification (ICD-9-CM) is an adaption created by the U.S. National Center for Health Statistics (NCHS) and used in assigning diagnostic and procedure codes associated with inpatient, outpatient, and physician office utilization in the United States. The ICD-9-CM is based on the ICD-9 but provides for additional morbidity detail. It is updated annually on October 1.[14][15]

Work on ICD-10 began in 1983, and the new revision was endorsed by the Forty-third World Health Assembly in May 1990. The latest version came into use in WHO Member States starting in 1994.[17] The classification system allows more than 155,000 different codes and permits tracking of many new diagnoses and procedures, a significant expansion on the 17,000 codes available in ICD-9.[18]
Adoption was relatively swift in most of the world. Several materials are made available online by WHO to facilitate its use, including a manual, training guidelines, a browser, and files for download.[3] Some countries have adapted the international standard, such as the "ICD-10-AM" published in Australia in 1998 (also used in New Zealand),[19] and the "ICD-10-CA" introduced in Canada in 2000.[20]

Adoption of ICD-10-CM was slow in the United States. Since 1979, the US had required ICD-9-CM codes[21] for Medicare and Medicaid claims, and most of the rest of the American medical industry followed suit. On 1 January 1999 the ICD-10 (without clinical extensions) was adopted for reporting mortality, but ICD-9-CM was still used for morbidity. Meanwhile, NCHS received permission from the WHO to create a clinical modification of the ICD-10, and has production of all these systems:

On 21 August 2008, the US Department of Health and Human Services (HHS) proposed new code sets to be used for reporting diagnoses and procedures on health care transactions. Under the proposal, the ICD-9-CM code sets would be replaced with the ICD-10-CM code sets, effective 1 October 2013. On 17 April 2012 the Department of Health and Human Services (HHS) published a proposed rule that would delay, from 1 October 2013 to 1 October 2014, the compliance date for the ICD-10-CM and PCS.[22] Once again, Congress delayed implementation date to 1 October 2015, after it was inserted into "Doc Fix" Bill without debate over objections of many.

Revisions to ICD-10-CM Include:

Relevant information for ambulatory and managed care encounter.

Expanded injury codes.

New combination codes for diagnosis/symptoms to reduce the number of codes needed to describe a problem fully.

Addition of sixth and seventh digit classification.

Classification specific to laterality.

Classification refinement for increased data granularity.

ICD-10-CA

ICD-10-CA is a clinical modification of ICD-10 developed by the Canadian Institute for Health Information for morbidity classification in Canada. ICD-10-CA applies beyond acute hospital care, and includes conditions and situations that are not diseases but represent risk factors to health, such as occupational and environmental factors, lifestyle and psycho-social circumstances.[20]

The World Health Organization has revised the International Classification of Diseases (ICD) towards the ICD-11. Its development has taken place on an internet-based workspace that continues to be used as the maintenance platform for discussions, and proposals for updates of ICD. Anybody can submit an evidence based proposal. The proposals are processed in an open transparent way with reviews for scientific evidence, and usability and utility in the various uses of ICD.
It is envisaged, that there will be no need for national modifications of ICD-11, due to its richness and flexibility in the reportable detail

ICD-11 comes with an implementation package that includes transition tables from and to ICD-10, a translation tool, a coding tool, web-services, a manual, training material, and more. All tools are accessible after self-registration from the maintenance platform.[23]

About 300 specialists of 55 countries, organized in 30 main working groups have provided their input to make ICD-11 scientifically up-to-date, and structural problems that became apparent using ICD-10 have been solved.

ICD-11 is much easier to use than ICD-10. Its ontological infrastructure allows for an improved user guidance compared to ICD-10.

The systematic reliance on the use of code combinations and extension codes makes ICD finally clinically relevant. With the combinations any condition can be coded to the desired level of detail.

Specialty versions, as for mental health, primary care, or dermatology are produced from the common core, the foundation.

ICD-11 is truly multilingual. ICD-10 exists in 43 Languages in electronic versions and ICD-11 has already 15 translations in progress. The tooling produces all files and formats from the core translation tooling on the maintenance platform . For authorization of translations, requests have to be made to WHO.

ICD-11 is digital health (formerly e-health) ready due to the use of uniform resource identifiers and its ontological underpinning. The system allows connection of any software through a standard API. The same package is also prepared for use offline.

ICD-11 is based on the electronic foundation component that contains all content, structural information, references and descriptions in a machine readable format. The content is then rendered for machine or human use, electronically or in print.

In ICD-11, each disease entity has a description that give key descriptions and guidance on what the meaning of the entity/category is in human-readable terms, to guide users. This is an advancement over ICD-10, which had only title headings. The Definitions have a standard structure according to a template with standard definition templates and further features exemplified in a "Content Model". The Content Model is a structured framework that captures the knowledge that underpins the definition of an ICD entity. The Content Model therefore allows computerization (with links to ontologies and SNOMED CT). Each ICD entity can be seen from different dimensions or "parameters". For example, there are currently 13 defined main parameters in the Content Model (see below) to describe a category in ICD.

An external review of the ICD-11 Revision has been completed. The report notes the progress in the ICD Revision, and makes clear recommendations about forward progress in the revision.[24]

ICD-11 invokes a more sophisticated architecture than historical versions, consistent with its generation as a digital resource. The core content of the system, called the Foundation Component, is a semantic network of words and terms, where any given term can have more than one parent. To address the requirement that statistical classifications exhibit mutual exclusiveness (so events are not counted more than once) and exhaustiveness (so there is a place to tally all events), ICD11 supports the serialization of the Foundation Component into an arbitrary number of linearizations, optimized for use cases. The main linearization, presently called the Joint Linearization for Morbidity and Mortality Statistics, is the tabular format with which most traditional users will become familiar. However, other linearizations, for primary care, multiple sub-specialty derivatives, or applications such as clinical decision support are possible. Finally, preliminary work in partnership with the IHTSDO is underway to ensure that the ICD-11 Foundation Component is semantically coherent through development of the Common Ontology, a subset of SNOMED CT which will anchor the Foundation Component to terms defined through description logic.

In the United States, the U.S. Public Health Service published The International Classification of Diseases, Adapted for Indexing of Hospital Records and Operation Classification (ICDA), completed in 1962 and expanding the ICD-7 in a number of areas to more completely meet the indexing needs of hospitals. The U.S. Public Health Service later published the Eighth Revision, International Classification of Diseases, Adapted for Use in the United States, commonly referred to as ICDA-8, for official national morbidity and mortality statistics. This was followed by the ICD, 9th Revision, Clinical Modification, known as ICD-9-CM, published by the U.S. Department of Health and Human Services and used by hospitals and other healthcare facilities to better describe the clinical picture of the patient. The diagnosis component of ICD-9-CM is completely consistent with ICD-9 codes, and remains the data standard for reporting morbidity. National adaptations of the ICD-10 progressed to incorporate both clinical code (ICD-10-CM) and procedure code (ICD-10-PCS) with the revisions completed in 2003. In 2009, the U.S. Centers for Medicare and Medicaid Services announced that it would begin using ICD-10 on April 1, 2010, with full compliance by all involved parties by 2013.[18]
However, the US extended the deadline twice and did not formally require transitioning to ICD-10-CM (for most clinical encounters) until October 1, 2015.

The years for which causes of death in the United States have been classified by each revision as follows:

The ICD includes a section classifying mental and behavioral disorders (Chapter V). This has developed alongside the Diagnostic and Statistical Manual of Mental Disorders (DSM) of the American Psychiatric Association and the two manuals seek to use the same codes. The WHO is revising their classifications in these sections as part the development of the ICD-11 (scheduled for 2018), and an "International Advisory Group" has been established to guide this.[27] Section F66 of the ICD-10 deals with classifications of psychological and behavioural disorders that are associated with sexual development and orientation. It explicitly states that "sexual orientation by itself is not to be considered a disorder," in line with the DSM and other classifications that recognise homosexuality as a normal variation in human sexuality. The Working Group has reported that there is "no evidence that [these classifications] are clinically useful" and recommended that section F66 be deleted for the ICD-11.[28]

An international survey of psychiatrists in 66 countries comparing use of the ICD-10 and DSM-IV found that the former was more often used for clinical diagnosis while the latter was more valued for research.[29] The ICD is actually the official system for the US, although many mental health professionals do not realize this due to the dominance of the DSM. A psychologist has stated: "Serious problems with the clinical utility of both the ICD and the DSM are widely acknowledged."[30]